Hydrophobic salts are a class of materials comprising electronically delocalized organic cations and polyatomic organic or inorganic anions. These salts, which typically possess melting points of less than 175° C., and more particularly, below 150° C., may also be liquid at room temperature and below.
In the 1940s Hurley and Wier first disclosed the preparation and use of ionic salts, which were fluid at room temperature, comprising the electronically delocalized N-alkylpyridinium cation and haloaluniinate inorganic anions in a series of U.S. Pat. (Nos. 2,446,331, 2,446,339, and 2,446,350). However, the practical utility of these haloaluminate-based pyridinium salts is severely compromised by their extremely high reactivity with trace amounts of water leading to the liberation of heat and toxic gases.
In the 1990s, air and water stable salts comprising the electronically delocalized 1,3-dialkylimidazolium cation and non-haloaluminate anions, e.g. BF4− and PF6− were prepared by Wilkes and coworkers (J Chem. Soc. Chem. Comm. 965 [1992]). However, these salts, because the alkyl groups on the imidazolium cation possessed less than three carbon atoms, have a significant solubility in water and may not be described as “hydrophobic”. Ellis describes (WO 9618459) a process for making ionic liquids by reacting a solution of a lead salt of an anion desired in the ionic liquid with 1,3-dialkylimidazolium halide salts, and separating the lead halide precipitate from an aqueous solution of the hydrophilic ionic liquid product. Finally, in U.S. Pat. No. 5,182,405 Arduengo describes a one-step process for the preparation of salts comprising 1,3-disubstituted imidazolium cations and the conjugate base of an acid. These salts, which are highly hydrophilic, are prepared via reaction of an alpha-dicarbonyl compound, an aldehyde (in toluene), an amine, and an acid comprising hydrogen halides, sulfric acid, or phosphoric acid.
The multi-step preparation of the more technologically useful hydrophobic salts comprising 1,3-dialkylimidazolium cations and the bis[trifluoromethylsulfonyl]imide anion was disclosed in U.S. Pat. No. 5,683,832 and in Inorg. Chem. 1168 (1996). U.S. Pat. No. 5,827,602 describes a multi-step reaction route leading to a broad spectrum of air and water stable hydrophobic salts comprising 1,3-dialkylimidazolium cations coupled with non-Lewis acid containing polyatomic anions possessing a van der Waals volume of greater than 100 A3.
However, all of the known procedures for preparation of hydrophobic salts minimally involve a two-step reaction sequence starting with an expensive 1-alkyllimidazole followed by alkylation by an alkyl halide. In addition to the high cost of the alkylimidazole, halide impurities (such as Cl−, Br−, and I−) from alkyl halide starting materials are both difficult and time consuming to remove. The purification step employing a silver reagent to produce halide-free hydrophobic salts is very expensive and unsuitable as an industrial process. Therefore, a need exists to improve the process for the preparation of ionic salts that are both hydrophobic and halide-free. The ability to inexpensively produce industrial quantities of such salts will enable the introduction of these technologically useful materials into a host of cost-sensitive chemical and engineering applications.